Diffuse radio emission in the merging cluster MACS?J0717.5+3745: the discovery of the most powerful radio halo

Context. Hierarchical models of structure formation predict that galaxy clusters grow via mergers of smaller clusters and galaxy groups, as well as through continuous accretion of gas. MACS?J0717.5+3745 is an X-ray luminous and complex merging cluster, located at a redshift of 0.5548. The cluster is suspected to host a bright radio relic, but up until now no detailed radio observations have been reported. Here we present Giant Metrewave Radio Telescope (GMRT) radio observations at 610?MHz of this cluster. Aims. The main aim of the observations is to study the diffuse radio emission within the galaxy cluster MACS?J0717.5+3745 related to the ongoing merger. Methods. We have carried out GMRT 610?MHz continuum observations of MACS?J0717.5+3745. These are complemented by Very Large Array (VLA) archival observations at 1.4, 4.9 and 8.5?GHz. Results. We have discovered a radio halo in the cluster MACS?J0717.5+3745 with a size of about 1.2?Mpc. The monochromatic radio power at 1400?MHz (P1.4) is 5 ? 1025?W?Hz-1, which makes it the most powerful radio halo known to date. A 700?kpc radio structure, which we classify as a radio relic, is located in between the merging substructures of the system. The global spectral index of radio emission within the cluster is found to be -1.24 ? 0.05 between 4.9?GHz and 610?MHz. We derive a value of 5.8??G for the equipartition magnetic field strength at the location of the radio halo. The location of the relic roughly coincides with regions of the intra-cluster medium (ICM) that have a significant enhancement in temperature as shown by Chandra. The major axis of the relic is also roughly perpendicular to the merger axis. This shows that the relic might be the result of a merger-related shock wave, where particles are accelerated via the diffuse shock acceleration (DSA) mechanism. Alternatively, the relic might trace an accretion shock of a large-scale galaxy filament to the south-east.